Last Update 06/ 06/ 2008
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The electrochemical cell represented below in the interactive scheme
has two compartments filled with their respective reagents in aqueous solution
at activity = 1.0. The temperature is 25.0 oC. One of the electrodes
is the standard hydrogen electrode in order to measure the standard equilibrium
potential of the cell. For introductory chemistry the concentrations of
the aqueous species may be sufficiently good if equal to 1.0 M. The required
hydrogen fugacity is 1.0. This may be acceptably replaced by an equal value
in atmosphere units. The two compartments are connected by a salt bridge
and two electrodes connected to an external circuit to a digital voltmeter.
Any good general chemistry book will inform on the several details of this
experiment. The components that take part in the chemical reaction defining
the cell are indicated in the respective cell compartment. The salt bridge
is here a potassium chloride solution kept inside an inverted U
tube. The voltmeter probe lead labeled C (common, also named earth),
is directed to compartment at the left side and the red colored led directed
to the other electrode. When the electrons try to flow from the electrode
at left to that at the right side and consequently from C to the
red connection, the voltage will have a positive sign. This indicates the
electric current i of the external circuit is running the reverse
path. The very high resistance of the voltmeter does not allow the electron
flow. When the voltmeter is substituted by another device with lower resistance
or by a suitable lamp, the effective electron flow will happen from the
electrode where the chemical reaction of oxidation occurs, named anode
to the compartment of reduction, named cathode. On the other hand, when
a conducting wire connects both electrodes the cell is said to be in short-circuit.
In this case the electron speed on the wire will be maximum.
A click on button A in the lower part of the interactive scheme
will substitute the experiment by another.
Exercise
You are invited to write the spontaneous chemical equation that would occur for the actual cell connected in short-circuit.
Please send your comments.
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